Conventionally, as an indexable ball-end mill, a two-blade type having two cutting inserts as disclosed in Patent Literature 1 is used in general. In the ball end mill described in the Literature, two types of cutting inserts, that is, a cutting insert functioning as a central cutting edge and a cutting insert functioning as an outer peripheral cutting edge are used. Each of the both cutting inserts includes a top surface, a bottom surface seated on an insert seat of the tool body, and a side surface connecting these surfaces. Two opposing cutting edges are provided on an intersecting ridge portion between the top surface and the side surface. The side surface (so-called flank) adjacent to the cutting edges is of a so-called positive type having a positive clearance angle.